1. Field of the Invention
This invention relates to a thread cutting device for a sewing machine, more particularly to a thread cutting device for cutting threads accurately.
2. Description of the Related Art
Referring to FIG. 1, a conventional thread cutting device for a sewing machine is shown to include a machine bed 1, a movable assembly 2, a thread-engaging needle 8, and a driving unit 9, such as a motor. The needle 8 is in front of the movable assembly 2. The machine bed 1 has a horizontal base plate 11, a driving rod 12 and a fixed stop member 13. The driving rod 12 is mounted pivotally on the base plate 11, and has a first end 121 connected pivotally to a shaft 91 of the driving unit 9, and a rounded second end 122. The movable assembly 2 includes an elongated horizontal feed plate 21, an elongated horizontal cutting plate 22 with a fixed cutter 23, a front crank 24, and a rear crank 25. The feed plate 21 is mounted slidably on a top surface of the base plate 11, and has a transversely extending projection 211. The horizontal cutting plate 22 is in friction contact with a top surface of the feed plate 21, and has a front longitudinal slide slot 221, a rear longitudinal slide slot 222, and an open-ended transverse slide slot 223. The cutter 23 is disposed at a front end of the cutting plate 22, and has two bladed barbs 231, 232. The front crank 24 has a pivot end 241 pivoted to the base plate 11, and a crankpin 242, which is received slidably within the front longitudinal slide slot 221 in the cutting plate 22. Similarly, the rear crank 25 has a pivot end 251 pivoted to the base plate 11, and a crankpin 252, which is received slidably within the front longitudinal slide slot 222 in the cutting plate 22. The rounded second end 122 of the driving rod 12 is received slidably within the transverse slide slot 223 in the cutting plate 22. Accordingly, when the driving rod 12 is swung by the driving unit 9, the second end 122 of the driving rod 12 pushes the cutting plate 22 to reciprocate on the base plate 11 along a curved path. At this time, the feed plate 21 is swung with the cutting plate 22 due to frictional contact therebetween. When the movable assembly 2 moves toward the thread-engaging needle 8, the projection 211 of the feed plate 21 will contact the stop member 13 of the base plate 11, thereby preventing further forward movement of the feed plate 21 on the base plate 11. Then, only the cutting plate 22 is pushed forward by the rounded second end 122 of the driving rod 12, thereby moving the cutter 23 to a front limit position indicated by the phantom lines along a curved path. As such, when moved rearward from the front limit position along the curved path, it is likely that the bladed barbs 231, 232 of the cutter 23 cannot engage a row of three threads 81 to be cut, which are located on one side of the needle 8. Accordingly, it is difficult for the cutter 23 to cut off all of the three threads 81.